1. Field of the Invention
The present invention relates to a manufacturing method for a semiconductor photoelectrochemical cell and a semiconductor photoelectrochemical cell, and in particular, to a manufacturing method for a semiconductor photoelectrochemical cell having photocatalyst effects and a semiconductor photoelectrochemical cell which is manufactured in accordance with this method.
2. Description of the Related Art
When light hits single crystal or microscopic particles of an N type oxide semiconductor such as titanium oxide in an electrolyte solution as shown in FIG. 5, electrons in a valence band (V.B) are excited and move to a conduction band (C.B). An oxidation reaction (Red→Ox) occurs in the vicinity of holes (h+) from which electrons have been removed in the valence band while a reduction reaction (Ox→Red) occurs in the vicinity of the conduction band where the excited electrons (e−) exist.
In the contact interface between the electrolyte solution and the semiconductor, however, the band curves due to a Schottky barrier, so that no reduction reaction occurs unless electrons move over this barrier. Therefore, only a slight amount of electrons contribute to the reduction reaction, and accordingly, the oxidation reaction occurs only slightly.
It was clarified in 1972, however, that electrolysis of water due to light can be induced by using platinum for the counter electrode in a semiconductor photoelectrochemical cell (NATURE Vol. 238, No. 5358, pp. 37-38 (1972)). In addition to this, it is known that similar effects can be gained by connecting platinum electrodes to titanium oxide single crystal or making microscopic particles of titanium oxide carry microscopic particles of platinum (CHEMICAL PHYSICS LETTERS Vol. 88, No. 1, pp. 50-54 (1982)). In this case, platinum is made to be carried using a method where a substance gained by reducing titanium oxide immersed in a platinic acid with formaldehyde is heated at a high temperature.
According to the above described prior art, however, in any event, expensive platinum is used for electrodes, and the manufacturing method for electrodes is also complicated, so that it is hard to say that the art is practical. After the above described findings, one of the present inventors developed a method according to which titanium is burned at 700° C. to 800° C. and thereby N type semiconductor having anatase type crystal can be generated, and titanium is burned at 1200° C. to 1500° C. and thereby N type semiconductor having rutile type crystal can be fabricated (Japanese Unexamined Patent Publication No. H6 (1994)-90824), buy in this case, efficient photocatalyst effects cannot be provided by mixing an appropriate amount of titanium metal in a titanium oxide layer that is formed on the surface after burning.
Furthermore, though an invention relating to a method for manufacturing a photocatalyst material by carrying out anodic oxidation on titanium metal and burning this in an atmosphere of 500° C. has been proposed (Japanese Unexamined Patent Publication No. 2000-271493), the preprocessing is complicated, and thus, this cannot be said to be a simple or practical method.